The present invention relates to radiographic X-ray imaging systems and, more particularly, to photoluminescent phosphors especially suitable for use in X-ray imaging systems at temperatures in excess of about 77.degree. kelvin.
X-ray imaging systems are useful for producing images of, for example, internal organs of a human body. The basic principle of their operation involves the passing of X-rays through an object of inquiry (e.g., a human body), which X-rays then impinge on a photoluminescent layer. The internal parts of the object of inquiry absorb some of the X-rays as a function of their structure, whereby the X-ray pattern impinging on the photoluminescent layer is representative of the structure of such internal parts.
The X-ray pattern on the photoluminescent layer stimulates a luminescent (or light) image in the photoluminescent layer. The material of the photoluminescent layer that luminesces is known as a phosphor. It is desirable that the phosphor exhibit a "fast-acting" response to stimulation by X-rays; that is, as used herein, a rapid response both in luminescing upon excitation by X-rays and ceasing to luminesce upon cessation of X-ray excitation. This is because typical X-ray imaging systems incorporate sophisticated and expensive digital computer apparatus that are most economically operated at high speed.
Typical known phosphors that are composed of ceramic exhibit a relatively slow-acting response to X-ray stimulation. This increases the expense of operation of a typical imaging system including complex digital computer apparatus that is costly to operate. It would thus be desirable to provide a phosphor that exhibits a fast-acting response to X-ray stimulation.
A further class of known phosphors are effective only at temperatures below about 77.degree. kelvin and are doped with luminescent activators. These phosphors comprise, for example: Cs.sub.2 ZrCl.sub.6 doped with one of Ir, Os, Re. Pt, Mo, Ru or U; Cs.sub.2 HfCl.sub.6 doped with Os, Mo or Re; and Cs.sub.2 SnCl.sub.6 doped with Ir, U, Re or Os. These doped phosphors are known to exhibit luminescence at about 4.4.degree. kelvin (liquid helium temperature), but not at about 77.degree. kelvin (liquid nitrogen temperature) or higher temperatures due to a parasitic effect within the host material. Accordingly, they are not suitable for use in typical X-ray imaging systems, at least without employing expensive cooling apparatus to maintain the required cold temperature.